The present invention relates to a testing method and apparatus for evaluating the mechanical durability of a catalytic converter assembly designed for mounting within the exhaust system of a motor vehicle.
Catalytic converter assemblies must provide a very high level of mechanical and thermal durability because of the harsh conditions of vibration and high temperature encountered in the automotive exhaust system environment. The assembly for a typical catalytic converter includes a catalyst support honeycomb mounted within a protective exterior enclosure, the honeycomb most often being composed of a refractory, high surface area ceramic material for effective support of an active catalyst. To protect the honeycomb from mechanical shock or vibration damage, it is typically supported within the enclosure by a layer of a refractory, resilient mounting material, this layer being pre-compressed in the course of converter assembly to apply a predetermined holding force to the exterior of the honeycomb. In general, it is the deterioration of this resilient layer through exhaust system vibration at high temperature that is the usual cause of honeycomb breakage and ultimate converter failure.
Many different vibration testing systems have been developed to test the resistance of mechanical and electrical components to physical damage under laboratory conditions. Typically, these systems employ vibration tables, activated by electrical or electromechanical means, on which the devices to be tested are mounted for exposure to controlled vibration. U.S. Pat. Nos. 5,083,463 and 5,641,910 are representative of the different table designs which have been developed for such systems.
The use of sensors to collect information about test conditions during vibration testing is also known. U.S. Pat. No. 4,539,845 to Molimar, for example, describes a device for fatigue-testing a mechanical component mounted between armatures activated by an electromagnetic vibrator wherein a displacement sensor is placed between the armatures to generate a sinusoidal feedback signal for controlling vibration conditions.
The design of vibration testing apparatus is of course dictated largely by the conditions to be encountered by the tested part in use. U.S. Pat. No. 4,445,381 to Russenberger, for example, describes a vibration testing apparatus for fatigue testing a part at low frequencies, mainly to avoid part heating that would affect fatigue performance. An important feature of the vibrator design of this apparatus is an arrangement of vibrator isolation springs, and elastomer rods within the vibrator electromagnetic oscillator, that make the resonance frequency of the vibrator independent of the elastic properties of the part under test. The development of higher frequency vibration modes in the system is also suppressed.
A test technique often used to assess the mechanical durability of catalytic converters is the hot vibration test. The hot vibration test is performed using a variety of methods. Most automobile companies have developed their own hot vibration test to simulate accelerated exposure. The test results are judged using a simple pass/fail criterion. Lacking quantitative data there is no possible method for establishing incremental design improvements or defining marginal system durability. That is, if a part fails there is no means of understanding how close it came to failing, or if it fails how close it came to passing.
The hot vibration test most often utilizes an engine as a source for hot gas, and an electrodynamic shaker table for simulating the vehicle vibration. The engines provide a highly variable source of input temperature. The lack of temperature control complicates assessment of the thermal gradient within the converter and consequently the system durability.
The hot vibration test is additionally commonly conducted using a single (e.g. 100 Hz) frequency sine wave vibration exposure. This is unrealistic because most automobiles produce a range of frequencies ranging from a few tens of hertz up to approximately 1000 Hz. In the case of motorcycle engines, vibration frequencies may range from 100 Hz to as high as 2000 Hz, and the need for converter mounting systems capable of withstanding even higher operating temperatures has been recognized. Despite these issues, however, the hot vibration test remains an industry standard for assessing converter durability regardless of its limitations.